The present invention relates to the use of ammonia and organic amines as catalysts in epoxy hybrid powder coatings.
Powder coatings are dry polymer powders that are coated onto a substrate and then heated to coalesce the particles and create the final coating. The technical demands on such coatings are significant: the powder must be able to be applied and cured to give a smooth coating which is strong and flexible. The binder resin must possess a rapid cure rate, at as low a temperature as possible, to allow high production throughput and minimum energy expenditure. Additionally, the powder must be non-tacky at temperatures up to 40xc2x0 C. so that the particles do not clump on prolonged storage before use.
Resin curing is often catalyzed by the addition of quaternary ammonium salt or phosphonium salt catalysts. In order to be effective, these salts must be compatible with the resin and have sufficient mobility in the resin melt to diffuse to and contact the reactive groups of the resin polymer components in order to catalyze the reaction. These catalysts introduce mobile ions into the resin coating that may compromise hydrolytic and electrochemical stability. Moreover, the act of blending the catalyst into the resin composition, typically by melt extrusion, can induce premature curing of the composition. Finally, a significant amount of catalyst by weight is required in the formulation due to the typically high molecular weight of the catalyst.
A significant portion of the powder coatings market is represented by carboxyl-functional polymer-epoxy hybrids, which are combinations of epoxy resins with carboxylic acid-ended oligomers, such as polyester oligomers. The use of quaternary ammonium or amine salts or phosphonium salts as catalysts for polyester-epoxy films is well known. These materials are generally added in either melt-blending or dry compounding before application to the article to be coated. However, generally larger amounts of amine or quaternary ammonium salts must be added to allow for full curing, resulting in excess materials to be used, which subsequently stay in the cured film as a potential impurity.
GB 1,474,140 discloses the use of nitrogenous bases, or thermally decomposable salts thereof, to catalyze the reaction of an oligomeric ester or a polyester containing free hydroxyl groups but substantially no free carboxyl groups with a compound containing at least two epoxy groups.
Japanese Patent Application 50-85632 describes a heat curable coating composition comprising a polyepoxy compound and a polycarboxy compound, at least one of which is a film-forming polymer, or a film-forming polymeric compound having both epoxy groups and carboxyl groups, in combination with ammonia or an organic amine; wherein at least some of the carboxyl groups are included in the form of a salt with the ammonia or organic amine. The compositions are prepared by conventional melt mixing of the components of the heat curable composition at 80-150xc2x0 C., followed by pulverizing the composition to a particle size of not more than about 200 xcexcm.
The present invention relates to a process for preparing heat curable coating compositions, comprising exposing a powder comprising a carboxyl-functional polymer and a polyepoxy compound to an amine chosen from the group consisting of organic amines and ammonia under mild conditions.
The present invention also relates to the products made by the process described herein.
It has been found that the rate of crosslinking in carboxyl-functional polymer-epoxy blends for powder coating can be greatly increased by catalytically reacting the carboxyl-functional polymer carboxylic acid end groups with ammonia or organic amines. Surprisingly, it has also been found that this reaction requires no additional compounding and may be carried out with pre-compounded epoxy-carboxyl-functional polymer powder by simply exposing the powder to ammonia or an organic amine, or mixtures thereof.
This exposure is done at a relatively mild temperature, i.e. a temperature that is not so high that it could induce premature cross-linking of the components of the coating composition. Temperatures between xe2x88x9230xc2x0 C. and +50xc2x0 C., and preferably between xe2x88x9210xc2x0 C. and +10xc2x0 C., are suitable for providing such mild conditions.
Suitable carboxyl-functional polymers for use in the process of this invention include carboxyl-functional polyester resins, carboxyl-functional polyacrylate resins, carboxyl-functional polymethacrylate resins, carboxyl-functional polyamide resins, carboxyl-functional polyimide resins and carboxyl-functional polyolefin resins. Preferred carboxyl-functional polymers are carboxylic-functional polyesters.
Suitable carboxyl functional polyester resins can be obtained by condensation polymerization of a dicarboxyl- or polycarboxyl-containing monomer with a dihydroxy or polyhydroxy monomer such that the carboxylic groups are in excess. Examples of suitable carboxyl-containing monomers are terephthalic acid, isophthalic acid, trimellitic acid, adipic acid, sebacic acid, maleic acid, and the like. Useful hydroxy compounds include ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol, dipropylene glycol, 1,4-cyclohexanedimethanol, neopentyl glycol, bis(hydroxyethyl)terephthalate, trimethylolpropane, glycerol, pentaerythritol, and the like.
Suitable carboxyl-functional acrylic resins can be obtained by polymerizing or copolymerizing a carboxyl-containing monomer such as acrylic acid, methacrylic acid or the like. Examples of monomers capable of copolymerizing with monomers such as acrylic acid, methacrylic acid and the like include acrylic acid esters, methacrylic acid esters, and other ethylene-based unsaturated monomers such as acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl acetate and the like.
Suitable carboxyl-functional polyolefin resins can be obtained by copolymerizing a carboxyl-containing monomer such as acrylic acid, methacrylic, or the like with an ethylenic monomer capable of copolymerization such as ethylene, propylene, butene, butadiene, chloroprene, vinyl chloride, styrene and the like.
Suitable carboxyl-functional polyamide resins can be obtained by condensation polymerization of a dicarboxyl- or polycarboxyl-containing monomer with a diamino or polyamino monomer such that the carboxylic groups are in excess. Examples of carboxyl-containing monomers are terephthalic acid, isophthalic acid, trimellitic acid, adipic acid, sebacic acid, maleic acid, and the like. Useful amino monomers include ethylenediamine, hexamethylenediamine, dodecamethylenediamine, xylylenediamine, m-phenylenediamine, p-phenylenediamine, and the like.
Suitable carboxyl-functional polyimide resins can be obtained by condensation polymerization of a biscarboxylic anhydride-containing monomer with a diamino or polyamino monomer such that the carboxylic groups are in excess. This can be accomplished either by using the bisanhydride monomer in excess or by effecting only partial condensation such that free carboxyl groups adjacent to amidized carboxyls remain un-imidized. Examples of carboxylic anhydride-containing monomers include pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride and biphenyl tetracarboxylic dianhydride. Useful amino monomers include ethylenediamine, hexamethylenediamine, dodecamethylenediamine, xylylenediamine, m-phenylenediamine, p-phenylenediamine, and the like.
Suitable polyepoxy compounds include epoxy compounds or resins having two or more glycidyl groups in one molecule. Such compounds, often used as curing agents, are a reaction product of a phenolic compound such as bisphenol A, phenol novolac, or the like with epichlorohydrin; a reaction product of a cresolic compound such as cresol novolac or the like with epichlorohydrin; a glycidyl ether obtained from a reaction of an alcoholic compound such as ethylene glycol, propylene glycol, 1,4-butanediol, polyethylene glycol, polypropylene glycol, neopentylglycol, glycerol, or the like with epichlorohydrin; a glycidyl ester obtained from a reaction of a carboxylic acid compound such as succinic acid, adipic acid, sebacic acid, phthalic acid, terephthalic acid, hexahydrophthalic acid, trimellitic acid, or the like with epichlorohydrin; 3,4-epoxy-6-methylcyclohexylmethyl 3,4-epoxy-6-methylcyclohexanecarboxylate, 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, or the like; triglycidyl isocyanurate (TGIC) and its derivatives; and the like. Preferred epoxy compounds are TGIC and the reaction product of bisphenol A and epichlorohydrin.
Organic amines suitable for the process of this invention include C1-C12 alkylamines, C6-C12 arylamines, C7-C12 alkarylamines, and C7-C12 aralkyamines. More specifically, suitable organic amines include primary amines such as methylamine, ethylamine, propylaamine, butylamine, ethylenediamine, methanolamine, ethanolamine, aniline, cyclohexylamine, benzylamine and the like; secondary amines such as dimethylamine, diethylamine, dipropylamine, dibutylamine, dimethanolamine, diethanolamine, diphenylamine, phenylmethylamine, phenylethylamine, dicyclohexylamine, piperazine, imidazole, 2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole, 2-isopropylimidazole, 2-phenylimidazole, 2-methylimidazoline, 2-phenylimidazoline and the like; and tertiary amines such as trimethylamine, triethylamine, dimethylhexylamine, N-methylpiperazine, dimethylbenzylamine, dimethylaniline and the like. The organic amines or ammonia can be used in the gaseous form or dissolved in a suitable aqueous or non-aqueous solvent. Aqueous ammonia and triethylamine are preferred.
The advantage of catalysis by ammonia is that the cation is highly mobile from the standpoint of diffusion through the polymer melt and may ultimately diffuse out of the polymer, leaving little or no catalyst residue in polymer. Also the catalyst is present only where it is needed, namely at the carboxylic ends where the reaction takes place with epoxides; this makes the catalysis much more efficient than the addition of conventional quaternary ammonium salt catalysts, which have much higher molecular mass and thus require a greater mass of catalyst to be effective. Finally, no persistent ionic species are introduced into the coating that might compromise hydrolytic and electrochemical stability. An additional advantage of this process is that it can enhance the xe2x80x9canti-blockingxe2x80x9d effect of the ammonia. The ammonium salt of some carboxyl-functional polymers, such as polyesters, have a higher softening temperature than the free carboxylic acid, which increases the temperature stability of the powder with regard to clumping. If the ammonium salt is present in higher concentrations at the surface of the particles than at the center of the particle, the anti-blocking effect should be enhanced for a given level of ammonia.
An illustrative example of a polyester ammonium salt-epoxy resin curing reaction is shown in Equations 1 and 2. 
The same benefits in cure rate enhancement are realized by reacting organic amines with the polymer carboxyl groups as are realized by reacting the carboxyl groups with ammonia, with the additional benefit in the former case of reducing yellowing in the cured coating. Conversion to the carboxyl-functional polymer alkylammonium salt may be accomplished by treating the powdered carboxyl-functional polymer-epoxy blend with an organic amine. Triethylamine is preferred.
The invention thus also provides a process for preparing a heat curable coating composition by exposing a powder that contains a carboxyl-functional polymer and a polyepoxy compound to a catalyst that is non-ionic, or is free, or substantially free, of ions, such as cations or anions. The catalyst may be a nitrogen-containing compound such as an organic amine or ammonia. The invention also provides a cured coating composition prepared from a powder that contains or is prepared from a carboxyl-functional polymer and a polyepoxy compound, wherein the curing of the composition is catalyzed by a nitrogen-containing compound, such as an organic amine or ammonia, and the composition is free, or substantially free, of catalyst residue, such as ionic catalyst residue, or is free, or substantially free, of ions, such as cations or anions. An article of manufacture may be coated by such composition.
While general practice for this invention is to expose the carboxyl-functional polymer-epoxy materials to ammonia and/or amine before application to a substrate to be coated, it is contemplated that the powder could be applied to the item, with subsequent exposure to ammonia and/or amine as described above.
Once the substrate has been coated with the powder, by any common means for applying powder coating to a substrate including but not limited to spray or fluidized bed application, it is generally baked to effect flow and cure of the powder. Generally, this curing is done at temperatures of 150xc2x0 C. and above. These substrates are used to make any number of articles of manufacturing, including, but not limited to, original equipment parts for automotive and other end-uses.